Cell property, adhesive property and coating property-controlled binder for lithium secondary battery with 2 or more phases

ABSTRACT

Disclosed is a binder comprising composite polymer particles having a structure formed of two or more phases different in terms of cell property, adhesive strength and/or coating property. The binder provides excellent cell property, adhesive strength and coating property, and thus the binder is used in an electrode of a lithium secondary battery to improve the cell performance.

TECHNICAL FIELD

The present invention relates to a binder for a lithium secondarybattery, preparation method thereof, a binder composition comprising thebinder, a slurry using the binder composition, an electrode formed ofthe slurry, and a lithium secondary battery prepared by using theelectrode.

BACKGROUND ART

Recently, developments for downsizing and weight reduction in portablecomputers, portable phones, camcorders, etc., are continuouslyprogressed. In view of this, a secondary battery used as a power sourceof the above electronic devices has a need of capacity increase,downsizing, weight reduction and thin film technology. Particularly, alithium secondary battery has advantages of a high voltage, a long life,a high energy density, etc., and thus, through active researches, hasbeen produced and sold.

The properties of a lithium secondary battery depend on the electrode,the electrolyte and other battery materials used therein. Among these,the physical properties of the electrode are determined by the binderthat provides bonding forces between active materials and collectors,and between active materials themselves. In particular, the higher theamount of the active material is, the higher the capacity of the cellproduct is, since the amount of the active material introduced to theelectrode relates to the maximum value of lithium ions which can bebound ultimately. Therefore, if the binder has an excellent adhesivestrength so as to reduce the amount of the binder introduced in theelectrode, the electrode having increased amount of active materials canbe produced. Accordingly, a binder having an excellent adhesive strengthis very much in demand.

The most typical binder currently used is a PVDF (polyvinylidenefluoride) polymer, which is used in the form of a binder composition bymixing with an organic solvent such as NMP (N-methyl-2-pyrrolidone).However, the PVDF-based binder has a disadvantage in that it isintroduced in a large amount in order to maintain a sufficient adhesivestrength, and it causes an environmental problem related with the use ofthe organic solvent, NMP. Accordingly, an attempt for making ahigh-efficiency binder composition using water as a dispersion medium isdisclosed (Japanese Patent Laid-Open Gazette No. Hei 5-21068, Hei5-74461, etc.), and a binder having a structure formed of two phasesdifferent in view of chemical structure is suggested (Korea PatentLaid-Open Gazette No. 2000-0075953). However, these binders of the priorarts cannot provide a sufficient adhesive strength between the activematerial and the collector, and thus have a problem in that the capacityis rapidly reduced by repetition of charge/discharge.

DETAILED DESCRIPTION OF THE INVENTION

By comparing the aforesaid binder preparation technologies according tothe prior arts, we have found that, when a binder has a structure formedof two or more phases different in view of cell property, adhesivestrength and coating property, the binder can provide a higher adhesivestrength, an excellent cell property and a better coating property ofslurry to be coated on the collector, wherein the binder can be preparedby polymerizing the detailed structure of the binder in separate two ofmore steps capable of controlling cell property, adhesive strength andcoating property.

According to an aspect of the present invention, it provides a binderfor battery comprising composite polymer particles having a structureformed of two or more phases different in terms of cell property,adhesive strength and/or coating property.

According to another aspect of the present invention, it provides amethod for preparing the binder comprising two or more phases havingdifferent physical properties as described above, a binder compositioncomprising the binder suspended in water or an organic solvent, a slurrycontaining the binder composition mixed with active materials andelectrode materials, an electrode for a lithium secondary battery formedof the slurry, and a lithium secondary battery obtained by using theelectrode.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following detailed description.

1. Binder for Battery (Composite Polymer Particles)

The binder according to the present invention comprises compositepolymer particles having a structure formed of two or more phases havingdifferent physical properties in terms of cell property, adhesivestrength and/or coating property, wherein the composite polymer particlecomprises: (a) a polymer based on monomers capable of controlling thecell property; and either or both of: (b) a polymer comprising monomerscapable of controlling the adhesive strength, and (c) a polymercomprising monomers capable of controlling the adhesive strength and thecoating property simultaneously.

The composite polymer particle forming the binder for battery accordingto the present invention is a single particle in which two or morepolymeric structures are differentiated by two or more different phases,not present in a single homogeneous phase and preferably interconnectedby chemical bonding. Preferably, the composite polymer particle havingtwo phases forms a core-shell structure, and the composite polymerparticle having three or more phases forms a three-dimensional structurelike an onion.

Additionally, the monomers forming the binder polymer are those thatcannot be dissolved in water or an organic solvent in the state of thepolymer thereof.

According to the present invention, the monomers forming the binderpolymer are divided into the monomers capable of controlling the cellproperty, the monomers capable of controlling the adhesive strength, andthe monomers capable of controlling the adhesive strength/coatingproperty. From the monomers divided as described above, the polymercapable of controlling the cell property, the polymer capable ofcontrolling the adhesive strength, and the polymer capable ofcontrolling the adhesive strength/coating property are separatelypolymerized in order to provide the binder for battery comprisingcomposite polymer particles having a structure formed of two or moredifferent phases.

The first group of monomers forming the polymer (a) capable ofcontrolling the cell property include: (1) a styrene-based monomer, forexample, styrene, α-methyl styrene, β-methyl styrene and p-t-butylstyrene; (2) ethylene and propylene; (3) a conjugated diene-basedmonomer, for example, 1,3-butadiene, 2,3-dimethyl-1,3-butadiene,1,3-pentadiene, p-perylene and isoprene; (4) a nitrile-containingmonomer, for example, acrylonitrile and methacrylonitrile; (5) anacrylic ester, for example, methyl acrylate, ethyl acrylate, propylacrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate,n-amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexylacrylate, hydroxypropyl acrylate and lauryl acrylate; (6) a methacrylicester, for example, aryl methacrylate, methyl methacrylate, ethylmethacrylate, propyl methacrylate, isopropyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamylmethacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate,hydroxypropyl methacrylate and lauryl methacrylate, or the like. Inaddition, the above polymer (a) is prepared by homopolymerizing orcopolymerizing the above first group of monomers. In the case ofcopolymerization, 2 to 10 kinds of monomers are preferably used.

The cell property described herein includes, for example, initialcapacity, initial efficiency and capacity change caused by repetition ofcharge/discharge, etc., and means the general evaluation about theseproperties.

Particularly, acrylonitrile monomer can improve the electricalproperties by the triple bond.

Each of the first group of monomers forming the polymer (a) capable ofcontrolling the cell property has the specific surface energy, and thepolymer formed from the monomers has a different value of surface energydepending on the mixing ratio of the monomers. The difference of thesurface energies can be represented by the difference of the contactangle of the polymer with the electrolyte, because it causes thedifference in the affinity with the electrolyte. Preferably, the contactangle of the polymer providing an excellent cell property with theelectrolyte is 60 degrees or less. Additionally, the mixing ratio ofeach monomer, although it is not particularly limited, is preferably0.01-70 parts by weight for the monomers (1) to (6), respectively, basedon 100 parts by weight of the polymer (a), and the compositionpreferably has a glass transition temperature of −10 to 30° C. and a gelcontent of 50% or more.

The second group of monomers forming the polymer (b) capable ofcontrolling the adhesive strength include functional monomers of: (1) anacrylamide-based monomer, for example, acrylamide, n-methylolacrylamideand n-butoxymethylacrylamide; (2) a methacrylamide-based monomer, forexample, methacrylamide, n-methylolmethacrylamide andn-butoxymethylmethacrylamide; (3) an unsaturated monocarboxylicacid-based monomer, for example, acrylic acid and methacrylic acid; and(4) an unsaturated dicarboxylic acid-based monomer, for example,itaconic acid, maleic acid, fumaric acid, citraconic acid, metaconicacid, glutaconic acid, tetrahydrophthalic acid, crotonic acid,isocrotonic acid and nadic acid, or the like.

The polymer (b) is prepared by homopolymerizing or copolymerizing one ormore monomers selected from the above second group of monomers, or bycopolymerizing one or more monomers selected from the above second groupof monomers with one or more monomers selected from the first group ofmonomers as described above. The polymer (b) obtained by coplymerizationpreferably comprises 2 to 15 kinds of monomers.

The second group of monomers forming the polymer (b) capable ofcontrolling the adhesive strength can improve the adhesive strength,because the functional group contained in each monomer has an excellentbonding force to the metal used as a collector. Accordingly, the mixingratio of each monomer, although it is not particularly limited, ispreferably 0.01-20 parts by weight for the second group of monomers (1)to (4), respectively, based on 100 parts by weight of the polymer (b),in order to control the adhesive strength.

The polymer (c) capable of controlling the adhesive strength and coatingproperty simultaneously can be prepared by copolymerizing: (1) theacrylamide-based monomer, particularly, acrylamide; (3) the unsaturatedmonocarboxylic acid-based monomer, particularly acrylic acid; or (4) theunsaturated dicarboxylic acid-based monomer, particularly itaconic acid,among the second group of monomers as described above, optionally withone or more additional monomers selected from the group consisting ofthe first group of monomers and the second group of monomers.

Among the second group of monomers, (1) the acrylamide-based monomer,(3) the unsaturated monocarboxylic acid-based monomer, or (4) theunsaturated dicarboxylic acid-based monomer provides different adhesivestrengths as well as different coating properties according to theposition occupied in the polymer by them. In particular, when themonomer is located at the outside of the polymer, it can control theadhesive strength and the coating property simultaneously.

The mixing ratio of each monomer, although it is not particularlylimited, is preferably 0.01-20 parts by weight for the monomers (1), (3)and (4), respectively, based on 100 parts by weight of the polymer (c),and it is possible to control the adhesive strength and the coatingproperty simultaneously by controlling the combination of monomers andthe location in the polymer.

In case of the composite polymer particles having a structure formed oftwo phases, the proportion of the polymer (a) is 50 to 90 wt % and thatof the polymer (b) or polymer (c) is 10 to 50 wt %, preferably. In thecase of the composite polymer particles having a structure formed ofthree phases, the proportion of the polymer (a) is 10 to 50 wt %, thatof the polymer (b) is 10 to 40 wt % and that of the polymer (c) is 10 to50 wt %, preferably. Additionally, in the case of the composite polymerparticles having a structure formed of four or more phases, it ispreferable that the proportion of the repeated polymerization of polymer(a) and polymer (b) is 50 to 90 wt % and the proportion of the polymer(c) is 10 to 50 wt %. Because it is preferable that a polymerpolymerized first is completely surrounded with a polymer polymerizedlater.

In addition, it is preferable that the composite polymer particles havethe final particle diameter ranged from 100 nm to 300 mn, the glasstransition temperature of each of the polymers (a), (b) and (c) isranged from −10° C. to 30° C. and the gel content is 50% or more.

Further, a molecular weight modifier and a crossliiking agent may beused as polymerization additives. Particularly, the amounts of themolecular weight modifier and the crosslinking agent introduced into thepolymerization can be controlled in order to control the gel content ofthe composite polymer particles. The molecular weight modifier that maybe used includes t-dodecylmercaptan, n-dodecylmercaptan,n-octylmercaptan, etc., the crosslinking agent that may be used includes1,3-butanediol diacrylate, 1,3-butanediol dimethacrylate, 1,4-butanedioldiacrylate, 1,4butanediol dimethacrylate, aryl acrylate, arylmethacrylate, trimethylolpropane triacrylate, tetraethyleneglycoldiacrylate, tetraethyleneglycol dimethacrylate, divinylbenzene, or thelike.

Any compound that causes the polymerization reaction may be used as apolymerization initiator, wherein the initiator compound includes, forexample, ammonium persulfate, potassium persulfate, benzoyl peroxide,azobisisobutyronitrile, butyl hydroperoxide, cumene hydroperoxide, etc.,and the water soluble or redox polymerizaiton initiators are morepreferable among them.

The composite polymer particles used in the present invention can beobtained by a conventional polymerization method, such as emulsionpolymerization, suspension polymerization, dispersion polymerization andseeded polymeriation. Generally, the temperature and the time for thepolymerization are about 50-200° C. and about 0.5-20 hours,respectively, although they may be optionally selected by thepolymerization method, the kind of the polymerization initiator, or thelike.

According to an embodiment of the present invention, the compositepolymer particles can be prepared by polymerizing the polymer (a) fromthe first group of monomers improving the cell property, and adding thesecond group of monomers improving the adhesive strength to the polymer(a) in order to polymerize the polymer (b) essentially comprising thesecond group of monomers into the structure having a phase differentfrom that of the polymer (a).

According to another embodiment of the present invention, the compositepolymer particles can be prepared by polymerizing the polymer (a) fromthe first group of monomers improving the cell property, andpolymerizing the polymer (c) improving the adhesive strength and thecoating property to the polymer (a) into the structure having a phasedifferent from that of the polymer (a).

According to another embodiment of the present invention, the compositepolymer particles can be prepared by polymerizing the polymer (a) fromthe first group of monomers improving the cell property, adding thesecond group of monomers improving the adhesive strength to the polymer(a) in order to polymerize the polymer (b) essentially comprised of thesecond group of monomers into the structure having a phase differentfrom that of the polymer (a), and polymerizing the polymer (c) improvingthe adhesive strength and the coating property thereto into thestructure having a phase different from that of the polymer (a) and thatof the polymer (b).

According to another embodiment of the present invention, the compositepolymer particles formed of four or more phases can be prepared bypolymerizing the polymer (a), the polymer (b) and the polymer (c),successively, wherein each of the steps for polymerizing the polymer(a), (b) and (c) is carried out two or more times using differentmonomers.

2. Binder Composition for Battery Electrode

The binder for a battery according to the present invention may bedissolved in a solvent or dispersed in a dispersion medium by aconventional method to form a binder composition.

The liquid material used as the dispersion medium for the bindercomposition of the present invention, although it is not particularlylimited, is preferably present in the liquid state at room temperatureunder the atmospheric pressure, so that the slurry for a batteryelectrode as described hereinafter can maintain the form of thecomposite polymer particles, when it is coated and dried on a collector.

Any dispersion medium capable of dispersing the said composite polymerparticles and active materials may be used, and the particular examplesof the dispersion medium includes, for example, water; alcohols such asmethanol, ethanol, propanol, isopropanol, butanol, isobutanol,s-butanol, t-butanol, pentanol, isopentanol and hexanol; ketons such asacetone, methyl ethyl ketone, methyl propyl ketone, ethyl propyl ketone,cyclopentanone, cyclohexanone and cycloheptanone; ethers such as methylethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutylether, diisobytyl ether, di-n-amyl ether, diisoamyl ether, methyl propylether, methyl isopropyl ether, methyl butyl ether, ethyl propyl ether,ethyl isobutyl ether, ethyl n-amyl ether, ethyl isoamyl ether andtetrahydrofuran; lactones such as γ-butyrolactone and δ-butyrolactone;lactames such as β-lactame; cycloaliphatic hydrocarbons such ascyclopentane, cyclohexane and cycloheptane; aromatic hydrocarbons suchas benzene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene,propylbenzene, isopropylbenzene, butylbenzene, isobutylbenzene andn-amylbenzene; aliphatic hydrocarbons such as heptane, octane, nonaneand decane; linear or cyclic amides such as dimethylformamide andN-methylpyrrolidone; esters such as methyl lactate, ethyl lactate,propyl lactate, butyl lactate and methyl benzoate; and liquid materialsforming the solvent for the electrolyte as described hereinafter, or thelike. Particularly, considering the process for producing the electrode,the dispersion medium having a boiling point of 80° C. or higher ispreferably used, with 85° C. or higher more preferably.

Additionally, if desired, other additives described hereinafterreferring to the slurry or other shelf stabilizers, etc., may be furtheradded.

3. Slurry for Battery Electrodes

The slurry of the present invention is obtained by mixing the bindercomposition of the present invention and active materials, optionallywith additives.

The electrode active materials are important in that they determine thecell capacity. The active materials used for cathode (positiveelectrode) include, for example, a conductive polymer such aspolypyrrol, polyaniline, polyacetylene and polythiophen, a metal oxidesuch as lithium cobalt oxide, lithium nickel oxide and lithium manganeseoxide, and a composite metal oxide formed of a metal oxide and anelectroconductive polymer. In addition, the active materials used foranode (negative electrode) include, for example, a carbonaceous materialsuch as a natural graphite, an artificaial graphite, MPCF, MCMB, PIC, asintered phenolic resin, a PAN-based carbon fiber and graphite, aconductive polymer such as polyacene, and a lithium-based metal such aslithium metal and a lithium alloy, or the like.

If desired, in addition to the active materials, a conductive agent, aviscosity modifier, a supplementary binder, etc., may be added to theelectrode slurry. The viscosity modifier that may be used includes awater soluble polymer, for example, carboxymethyl cellulose,carboxyethyl cellulose, ethyl cellulose, hydroxymethyl cellulose,hydroxypropyl cellulose, carboxyethylmethyl cellulose, polyethyleneoxide, ethylene glycol, or the like.

4. Electrode for Lithium Secondary Battery

The electrode of the present invention is obtained by coating the slurryfor battery electrode on a collector and removing the dispersion mediumby drying, etc. in order to bond the active materials to the collectorand to bond the active materials themselves.

In general, a collector formed of a metal such as iron, copper,aluminum, nickel, etc., may be used, although there is no specificlimitation as long as a collector formed of a conductive material isused.

5. Lithium Secondary Battery

The lithium secondary battery of the present invention comprises theelectrode of the present invention as described above as the positiveelectrode and/or negative electrode. The electrolyte solution of thislithium secondary battery may be a conventional electrolyte solution,and the electrolyte having a function as a battery depending on thekinds of the negative electrode active materials and the positiveelectrode active materials may be selected. For example, for theelectrolyte of the lithium secondary battery, LiPF₆, LiClO₄, LiBF₄,LiN(SO₂CF₃)₂, etc., may be used, and for the solvent, mixture of anyhigh-dielectric solvents such as EC or PC and any low-viscosity solventssuch as alkyl carbonates(DEC, DMC, EMC, etc.) may be used.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in detail by using the followingexamples. However, the following examples are for illustrative purposesonly and the present invention is not limited thereto.

EXAMPLE 1

[Binder Composition]

196.0 g of deionized water was introduced into a reactor and thetemperature was increased to 75° C. When the temperature of doionizedwater reached to 75° C., 49.8 g of styrene, 46.5 g of 1,3-butadiene and0.65 g of sodium lauryl sulfate were added thereto. And then, whilemaintaining the temperature inside the reactor at 15° C., 0.32 g ofpotassium persulfate dissolved in 10.0 g of deionized water wasintroduced to complete the polymerization of polymer (a).

To the polymer (a), an emulsified mixture of 93.0 g of deionized water,30.0 g of styrene, 60.1 g of butyl acrylate, 0.8 g of aryl methacrylate,5.4 g of itaconic acid and 0.15 g of sodium lauryl sulfate wereintroduced in portions for 3 hours, while 0.21 g of potassium persulfatedissolved in 10.0 g of deionized water was also introduced in portionsfor 3 hours to complete the polymerization of polymer (b).

The pH of the composite polymer product obtained as described above wasadjusted to pH 7 using potassium hydroxide to form a binder compositionfor an anode. And then, 500 g of NMP was added to 50 g of the bindercomposition and water was removed by distillation at 90° C. in order toform a binder composition for cathode.

The specific physical properties of the polymerized binder weredetermined in three categories as follows. First, the particle diameterdetermined by a light scattering system was 161 nm and the glasstransition temperature determined by DSC (Differential ScanningCalorimeter) at scanning rate of 10° C./min was −3° C. In addition, thegel content determined by using toluene as a solvent was 85%.

[Slurry]

94 g of a natural graphite, 1.0 g of a conductive polymer, 2.5 g of thebinder and 2.5 g of a water-soluble polymer were mixed to the totalsolid content of 45% by using water as a dispersion medium in order toform the slurry for an anode.

Additionally, 94 g of LiCoO₂, 1.0 g of a conductive polymer and 5.0 g ofthe binder were mixed to the total solid content of 45% by using NMP asa dispersion medium in order to form the slurry for a cathode.

[Electrode]

The slurry for anode was coated on a copper foil and the slurry forcathode was coated on an aluminum foil, in the thickness of 200 μm ineach of cases, and then dried at 90° C. for 10 minutes and at 120° C.for 10 minutes at the atmospheric pressure, and at 120° C. for 2 hoursunder vacuum. Each of the dried electrodes was pressed at the porosityof 30% to form the electrodes completely.

[Cell]

A separator formed of polyolefin microporous membrane was insertedbetween the dried cathode and anode to provide a coin-type cell. Then,electrolyte solution obtained from LIPF₆ electrolyte dissolved in amixed solvent consisting of EC:EMC=1:2 (volume ratio) at theconcentration of 1 mol/l, was introduced to form the cell completely.

[Evaluation for the Cell Performance]

For the evaluation of the cell property, 3 cycles and 30 cycles ofcharge/discharge were repeated by static current method at 0.1 C,wherein initial capacity, initial efficiency, the capacity after 3cycles and the capacity after 30 cycles were compared. The evaluationresult was defined as the average obtained by forming 5 or morecoin-type cells for the same binder composition and evaluating theirperformances.

[Evaluation for the Adhesive Strength]

In order to determine the adhesive strength between active materials anda collector, after attaching epoxy plate to the surface of the aforesaidelectrode to fix the active materials, the collector cut into a constantthickness was peeled to determine 180° peel strength. The evaluationresult for the peel strength was defined in the average of 5 or moretimes of determination values.

[Evaluation for the Coating Property]

In order to evaluate the coating property, slurry having the solidcontent increased from the original 45% to 51% was fonned. Similarly,the slurry was coated on a collector at the thickness of 200 μm and thecoating state was signified by the signs “O” and “X”, wherein “O” meansa state in which the slurry was completely coated on the collector, and“X” means a state in which a portion of surface not coated with theslurry was present.

EXAMPLE 2

[Binder Composition]

The same polymerization method as in EXAMPLE 1 was repeated to formpolymer (a), and then an emulsified mixture of 93.0 g of deionizedwater, 30.0 g of styrene, 60.1 g of butyl acrylate, 0.8 g of arylmethacrylate, 2.0 g of itaconic acid, 1.4 g of acrylamide, 2.0 g ofacrylic acid and 0.15 g of sodium lauryl sulfate were introduced to thepolymer (a) in portions for 3 hours, while 0.21 g of potassiumpersulfate dissolved in 10.0 g of deionized water was introduced inportions for 3 hours to complete the polymerization of polymer (c).

The binder composition was prepared by the same method as in EXAMPLE 1,and the same processes for determining the specific physical propertiesas in EXAMPLE 1 were repeated. As the result, the particle diameter was158 nm, the glass transition temperature was −5° C., and the gel contentwas 86%.

In addition, the method for preparing slurry from the bindercomposition, the cell production method, the evaluation for the cellperformance, the evaluation for the adhesive strength and coatingproperty, were the same as in EXAMPLE 1.

EXAMPLE 3 to EXAMPLE 7

EXAMPLE 1 or EXAMPLE 2 was repeated, except that the monomers used forthe binder composition were varied as described in the following Table 1to polymerize the composite polymer particles formed of polymer (a) andpolymer (b), or polymer (a) and polymer (c).

Other methods for preparing the binder composition, determining thespecific physical properties, preparing the slurry, forming the cell,and evaluating the cell performance, the adhesive strength and thecoating property were performed as described in EXAMPLE 1. TABLE 1<expressed in g units> EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 EXAMPLE 6 EXAMPLE 7Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer PolymerPolymer (a) (a) (a) (b) (a) (c) (a) (c) (a) (c) Styrene 49.8 31.8 31.847.0 25.0 30.0 25.0 30.0 31.8 47.0 Methyl 24.8 methacrylateAcrylonitrile 24.8 Aryl methacrylate 0.8 0.8 0.8 0.8 0.8 Butyl acrylate63.7 63.7 60.1 60.1 63.7 1,3-butadiene 46.5 43.9 46.5 46.5 43.9Acrylamide 1.4 1.4 1.4 Itaconic acid 5.4 2.0 2.0 2.0 Acrylic acid 2.02.0 2.0 Particle diameter 155 163 160 161 158 (nm) Glass transition −4−4 −3 −5 −4 temperature (° C.) Gel content (%) 85 83 85 86 85

COMPARATIVE EXAMPLES 1 and 2, and EXAMPLES 8 and 9

EXAMPLE 1 or EXAMPLE 2 was repeated, except that the monomers used forthe binder composition were varied as described in the following Table 2to polymerize the composite polymer particles with two, three or fourphases. However, in order to control the sizes of the composite polymerparticles uniformly, the amount of sodium lauryl sulfate introduced tothe polymerization of polymer (a) was changed to 0.23 g, 0.71 g and 1.1g, respectively.

Other methods for preparing the binder composition, determining thespecific physical properties, preparing the slurry, forming the cell andevaluating the cell performance, the adhesive strength and the coatingproperty were performed as described in EXAMPLE 1. TABLE 2 <expressed ing units> COMP. COMP. EX. 1 EX. 2 EXAMPLE 8 EXAMPLE 9 Polymer PolymerPolymer Polymer Polymer Polymer Polymer Polymer Polymer (c) (c) (a) (b)(c) (a) (a) (b) (c) Styrene 30.0 47.0 25.0 30.0 30.0 25.0 25.0 30.0 30.0Acrylonitrile 24.8 24.8 24.8 Aryl methacrylate 0.8 0.8 0.8 0.8 0.8 Butylacrylate 60.1 60.1 60.1 60.1 60.1 1,3-butadiene 43.9 46.5 46.5 46.5Acrylamide 1.4 1.4 1.4 1.4 Itaconic acid 2.0 2.0 5.4 2.0 5.4 2.0 Acrylicacid 2.0 2.0 2.0 2.0 Particle diameter (nm) 164 153 167 168 Glasstransition −2 −4 −3 −4 temperature (° C.) Gel content (%) 86 84 84 83

EXAMPLE 10 to EXAMPLE 14

EXAMPLE 2 was repeated, except that the monomers used to polymerize thecomposite polymer particles were varied as described in the followingTable 3, in order to modify the glass transition temperature and gelcontent of the composite polymer particles.

Other methods for preparing the binder composition, determining thespecific physical properties, preparing the slurry, forming the cell,and evaluating the cell performance, the adhesive strength and thecoating property were performed as described in EXAMPLE 1. TABLE 3<expressed in g units> EXAMPLE 10 EXAMPLE 11 EXAMPLE 12 EXAMPLE 13EXAMPLE 14 Polymer Polymer Polymer Polymer Polymer Polymer PolymerPolymer Polymer Polymer (a) (c) (a) (c) (a) (c) (a) (c) (a) (c) Styrene25.0 43.0 25.0 56.0 25.0 30.0 25.0 30.0 25.0 30.0 t-dodecyl 0.3 0.8mercaptane Acrylonitrile 24.8 24.8 24.8 24.8 24.8 Aryl methacrylate 0.80.8 2.0 0.8 0.8 Butyl acrylate 47.1 34.1 60.1 60.1 60.1 1,3-butadiene46.5 46.5 46.5 46.5 46.5 Acrylamide 1.4 1.4 1.4 1.4 1.4 Itaconic acid2.0 2.0 2.0 2.0 2.0 Acrylic acid 2.0 2.0 2.0 2.0 2.0 Particle diameter158 163 160 161 158 (nm) Glass transition 14 37 −3 −5 −4 temperature (°C.) Gel content (%) 85 83 95 63 44

EXAMPLE 15 to EXAMPLE 17

The same formulation as in EXAMPLE 6 was used, except that the amount ofsodium lauryl sulfate introduced to the polymerization of polymer (a)was changed to 1.15 g, 0.3 g and 0.18 g, respectively, as described inthe following Table 4, in order to modify the particle size of thecomposite polymer particles.

Other methods for preparing the binder composition, determining thespecific physical properties, preparing the slurry, forming the cell,and evaluating the cell performance, the adhesive strength and thecoating property were performed as described in EXAMPLE 1. TABLE 4<expressed in g units> EXAMPLE 15 EXAMPLE 16 EXAMPLE 17 polymer polymerpolymer polymer polymer polymer (a) (c) (a) (c) (a) (c) Styrene 25.030.0 25.0 30.0 25.0 30.0 t-dodecyl mercaptane Acrylonitrile 24.8 24.824.8 Aryl 0.8 0.8 0.8 methacrylate Butyl acrylate 60.1 60.1 60.11,3-butadiene 46.5 46.5 46.5 Acrylamide 1.4 1.4 1.4 Itaconic acid 2.02.0 2.0 Acrylic acid 2.0 2.0 2.0 Particle 93 219 314 diameter (nm) Glasstransition −5 −3 −3 temperature (° C.) Gel content 85 83 86 (%)

[Evaluation Results]

The results of the evaluation for the cell performance, the adhesivestrength and the coating property obtained from the above EXAMPLES 1-17and COMPARATIVE EXAMPLES 1 AND 2 are summarized in the following Table5. TABLE 5 Cell performance Initial Initial Capacity Capacity adhesivecapacity efficiency (3 cycles) (30 cycles) strength Coating (mAh/g) (%)(mAh/g) (mAh/g) (g/cm) property EXAMPLE 1 299 87.8 280 247 20.3 XEXAMPLE 2 298 88.0 278 245 21.2 ◯ EXAMPLE 3 298 87.9 280 247 7.2 XEXAMPLE 4 297 87.9 281 248 18.8 X EXAMPLE 5 301 88.3 283 252 21.3 ◯EXAMPLE 6 307 89.6 295 272 21.8 ◯ EXAMPLE 7 297 87.8 280 246 19.7 ◯COMP. EX. 1 294 87.5 276 241 21.3 ◯ COMP. EX. 2 295 87.5 279 245 19.8 ◯EXAMPLE 8 305 89.8 291 269 21.2 ◯ EXAMPLE 9 303 89.6 286 268 21.4 ◯EXAMPLE 309 89.7 296 272 24.7 ◯ 10 EXAMPLE 310 89.8 298 274 19.2 ◯ 11EXAMPLE 308 89.6 296 272 21.8 ◯ 12 EXAMPLE 308 89.8 298 273 19.9 ◯ 13EXAMPLE 309 89.7 298 275 15.3 ◯ 14 EXAMPLE 308 89.7 298 273 19.6 ◯ 15EXAMPLE 307 89.8 297 275 23.7 ◯ 16 EXAMPLE 309 89.7 297 275 20.1 ◯ 17

As shown in Table 5, comparative examples 1 and 2 using the bindercomprised of polymer (c) alone show a low cell property due to theabsence of the component of polymer (a) capable of improving the cellproperty. Compared with this, the other examples using the bindercomprised of composite polymer particles having a structured form of twoor more phases including polymer (a) phase, have a tendency to improvethe cell property.

In particular, when the component of polymer (a) comprisesacrylonitrile, the cell property was greatly improved. It seems thatthis is because the triple bond in acrylonitrile improves the electricalproperty.

In addition, the adhesive strength was particularly increased, when thecomponent of polymer (c) comprises a second group of monomers such asitaconic acid, acrylic acid or acrylamide. It seems that this is becaudeof the excellent bonding force between the functional groups containedin the second group of monomer component and a collector.

Further, when the glass transition temperature ranged from −10° C. to30° C., the gel content was 50% or more and the particle size rangesfrom 100 nm to 300 nm, the adhesive strength was remarkably improved.

When the component of polymer (b) or polymer (c) comprises a controlledamount of a second group of monomer component such as itaconic acid,acrylic acid or acrylamide, the coating property was excellent. However,when the second group of monomer was not used, the coating property wasdecreased.

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, according to the present invention,the binder having a structure formed of two or more phases, a particlediameter ranged from 100 nm to 300 nm, a glass transition temperatureranged from −10° C. to 30° C. and the gel content of 50% or moreprovides excellent adhesive strength, cell property and coating propertyas compared with a conventional binder.

Therefore, the binder according to the present invention may be utilizedin a process for manufacturing a lithium secondary battery to improvethe productivity and to provide a lithium secondary battery having anexcellent cell property.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment, but, on the contrary, it is intended to covervarious modifications and variations within the spirit and scope of theappended claims.

1. A binder comprising composite polymer particles having a structuredform of two or more phases having different physical properties in termsof cell property, adhesive strength and/or coating property, wherein thecomposite polymer particle comprises: (a) a polymer based on monomerscapable of controlling the cell property; and either or both of: (b) apolymer based on monomers capable of controlling the adhesive strength,and (c) a polymer based on monomers capable of controlling the adhesivestrength and the coating property simultaneously.
 2. The binderaccording to claim 1, wherein the composite polymer particle comprisesthe polymer (a), the polymer (b) and the polymer (c) in turn, startingfrom the inside of the binder.
 3. The binder according to claim 1,wherein the inside polymer is surrounded with the outside polymer andthe physical properties of both polymers are different.
 4. The binderaccording to claim 1, wherein the polymer (a) is polymerized from one ormore monomers selected from the first group of monomers consisting ofstyrene-based monomers, ethylene, propylene, conjugated diene-basedmonomers, nitrile-containing monomers, acrylic esters and a methacrylicesters, among the monomers forming the binder polymer.
 5. The binderaccording to claim 4, wherein the first group of monomers comprise:styrene, α-methyl styrene, β-methyl styrene, p-t-butyl styrene;ethylene, propylene; 1,3-butadiene, 2,3-dimethyl-1,3-butadiene,1,3-pentadiene, p-perylene, isoprene; acrylonitrile, methacrylonitrile;methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate,n-butyl acrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate,n-hexyl acrylate, 2-ethylhexyl acrylate, hydroxypropyl acrylate, laurylacrylate; methyl methacrylate, ethyl methacrylate, propyl methacrylate,isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate,n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate,2-ethylhexyl methacrylate, hydroxypropyl methacrylate and laurylmethacrylate.
 6. The binder according to claim 1, wherein the polymer(b) is homolymerized or copolymerized from one or more monomers selectedfrom the second monomer group consisting of acrylamide-based monomers,methacrylamide-based monomers, unsaturated monocarboxylic acid-basedmonomers and unsaturated dicarboxylic acid-based monomers, or iscopolymerized from one or more monomers selected from the second monomergroup and one or more monomers selected from the first group of monomersas defined in claim
 4. 7. The binder according to claim 6, wherein thesecond group of monomers comprise: acrylamide, n-methylolacrylamide,n-butoxymethylacrylamide; methacrylamide, n-methylolmethacrylamide,n-butoxymethylmethacrylamide; acrylic acid, methacrylic acid; itaconicacid, maleic acid, fumaric acid, citraconic acid, metaconic acid,glutaconic acid, tetrahydrophthalic acid, crotonic acid, isocrotonicacid and nadic acid.
 8. The binder according to claim 1, wherein thepolymer (c) is copolymerized from acrylamide-based monomers, unsaturatedmonocarboxylic acid-based monomers and unsaturated dicarboxylicacid-based monomers among the monomers forming the binder polymer. 9.The binder according to claim 8, wherein the polymer (c) iscopolymerized with one or more additional monomers selected from thefirst group of monomers as defined in claim 4 and the second group ofmonomers as defined in claim
 6. 10. The binder according to claim 8,wherein the polymer (c) is a copolymer comprising acrylamide, acrylicacid and itaconic acid.
 11. The binder according to claim 1, wherein thebinder comprises the composite polymer particles formed of four or morephases prepared by polymerizing the polymer (a), polymerizing thepolymer (b) and polymerizing the polymer (c), successively, in whicheach of the steps for polymerizing the polymer (a), (b) and (c) iscarried out in two or more times using different monomers.
 12. Thebinder according to claim 1, wherein the composite polymer particleshaving a structured form of two phases comprises 50 to 90 wt % of thepolymer (a) and 10 to 50 wt % of the polymer (b) or polymer (c), thecomposite polymer particles having a structured form of three phasescomprises 10 to 50 wt % of the polymer (a), 10 to 40 wt % of the polymer(b) and 10 to 50 wt % of the polymer (c), and the composite polymerparticles having a structure of four or more phases comprises 50 to 90wt % of the repeated polymerization of polymer (a) and polymer (b) and10 to 50 wt % of the polymer (c).
 13. The binder according to claim 1,wherein the final particle size ranges from 100 nm to 300 nm.
 14. Thebinder according to claim 1, wherein the glass transition temperature ofeach of polymer (a), polymer (b) and polymer (c) ranges from −10° C. to30°.
 15. The binder according to claim 1, wherein the gel content is 50%or more.
 16. A slurry for an electrode of a lithium secondary batterycomprising the binder as defined in claim 1 and active materials.
 17. Anelectrode for a lithium secondary battery obtained by coating the slurryas defined in claim 16 on a collector.
 18. A lithium secondary batterycomprising the electrode as defined in claim 17.